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7Motion Perception
7 Motion Perception
• computation of visual motion
• eye movements
• using motion information
• akinotopsia
• http://www.sinauer.com/wolfe/chap7/mottypesF.htm
7 Computation of Visual Motion
• How would you build a neural motion detector?
– (Reichart 1950’s)
• Step1
– two adjacent receptors
– small distance apart
• Step 2
– intermediary delay neuron
7 A Neural Circuit for Detection of Rightward Motion (Part 1)
7 A Neural Circuit for Detection of Rightward Motion (Part 2) 7 Computation of Visual Motion (cont’d)
• Apparent motion:
– illusory impression of smooth motion resultingfrom the rapid alternation of objects that appear indifferent locations in rapid succession
– Demo 1 - web activity
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7 Computation of Visual Motion (cont’d)
• Apparent motion demonstrated first by
– Sigmund Exner (1875)
• neural circuit does not need real motion inorder to fire
• What is required
– discrete time locked samples of images
– change between two images
» That is small enough to “correspond”
7 Still Images from an Animation
7 Computation of Visual Motion (cont’d)
• Related problems with neural detection
– correspondence problem (motion):
• problem faced by the motion detection system ofknowing which feature in frame 2 corresponds to aparticular feature in frame 1
– spokes on a wheel demo
– aperture problem:
• when a moving object is viewed through an aperture(receptive field), the direction of motion of a local featureor part of the object may be ambiguous
– web activity demo
7 Global Motion Detector
7 Computation of Visual Motion (cont’d)
• Neural solution to aperature problem
– global-motion detectors:
• lesions in magnocellular layers of LGN impairperception of large, rapidly moving objects
– middle temporal lobe:
• plays important role in motion perception
• MT neurons selective for direction of motion
7 The Medial Temporal Lobe
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7 Computation of Visual Motion (cont’d)
• Neurophsyiological experiment (Newsome and Pare, 1988)
– trained monkeys to respond to correlated dotmotion displays (demo)
– lesion MT areas of monkeys
– results:
• monkeys needed about 10 times as many dotsto correctly identify direction of motion
7 The Newsome and Pare Paradigm
7 Computation of Visual Motion (cont’d)
• disadvantages of lesion studies :
– invasive
– lesions may be incomplete or may influence otherstructures
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• Saltzman, et al. (1990)
– trained monkeys on the correlated motion task
– found MT neurons sensitive to motion direction
– ambiguous cases
• stimulated neurons
• biased the direction response
7 Computation of Visual Motion (cont’d)
• Motion aftereffect:
– illusion of a stationary object that occurs afterprolonged exposure to a moving object
• existence of this effect implies an opponent-process system, like that of color vision
• interocular transfer:
– transfer of an effect (e.g., adaptation) fromone eye to the other
7 Computation of Visual Motion (cont’d)
• What does interocular transfer tell us about the locusof the MAE in the visual system?
– result of activities of neurons in a part of the visualsystem where information collected from two eyesis combined (i.e., beyond LGN)
– input from both eyes is combined in area V1
– recent studies (fMRI) Huk et al. (2001):
• locate site of motion aftereffects more precisely
• MT
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7 Computation of Visual Motion (cont’d)
• first-order motion:
– motion of an object defined by luminance change
• second order motion (web demo):
– motion of an object that is defined by changes incontrast or texture, but not by luminance
• demo
– double dissociation
• 1st and 2nd order motions (Vaina et al. 96; 98)
7 Second-Order Motion (Part 1)
7 Second-Order Motion (Part 2) 7 Eye Movements
• Eye movements:
– smooth pursuit:
• eyes move smoothly to follow moving object
– saccade:
• rapid movement of eyes that change fixationfrom one object or location to another
– superior colliculus:
• structure in midbrain that plays important role ininitiating and guiding eye movements
7 Smooth Pursuit 7 Eye Movements (cont’d)
• Why do we perceive the pencil to be in motion in thefirst case, but perceive the dot to be stationary in thesecond case?
– because in one case there is an eye movement
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7 Eye Movements (cont’d)
• 6 muscles attached to each eye
– arranged in three pairs:
• controlled network of structures in the brain
– when stimulated with electrical signals, eyemovements can be observed
7 Physiology of the Eye (Part 1)
7 Physiology of the Eye (Part 2) 7 Eye Movements (cont’d)
• vergence eye movements:
– two eyes move in opposite directions, donedeliberately
• saccades:
– two eyes move in the same direction
• voluntary eye movements
7 Eye Movements (cont’d)
• saccadic suppression:
– reduction of visual sensitivity during a saccadiceye movement;
– eliminates blur from retinal image motion during aneye movement
7 Eye Movements (cont’d)
• motor system solves “problem” of why an object inmotion may appear stationary by sending out twocopies of each order to move eyes
– one copy goes to eye muscles
– another (“efference copy”) goes to an area ofvisual system that has been dubbed “comparator”
• comparator can then compensate for imagechanged caused by eye movement, inhibitingany attempts by other parts of the visualsystem to interpret changes as object motion
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7 The Comparator 7 Using Motion Information
• How do we use motion information to navigate?
– optic array:
• describes collection of light rays that interactwith objects of the world in front of viewer
– optic flow:
• changing angular position of points inperspective image that you experience as youmove through the world
– e.g., “radial expansion”
7 Optic Flow 7 Using Motion Information (cont’d)
• Are humans actually able to make use of optic flowinformation?
– Computer-generated displays of moving dots andlines to stimulate optic flow information
http://www.reading.ac.uk/arl/clips/demo_of_displays.htm#opticflow
7 Using Motion Information (cont’d)
• Biological motion:
– pattern of movement of living things (i.e., humans,animals)
• Demo 1
• Demo 2
7 Biological Motion
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7 Using Motion Information (cont’d)
• Avoiding imminent collision: how do we estimate thetime to collision (TTC) of an approaching object?
– Information source:
• Tau (τ);
– relies on information in retinal image
» track visual angle subtended byapproaching object over time
– Not clear if we actually make use of tau
7 The Man Who Couldn’t See Motion
• Akinetopsia:
– rare neuropsychological disorder in which theaffected individual has no perception of motion
7 The Man Who Couldn’t See Motion (cont’d)
• A 47-year-old man
– sees streams of multiple, frozen images trailing inthe wake of moving objects.
• when motion ceases, the images collapsed into eachother.
• When nothing is moving and he held perfectly still, hisvision was entirely normal.
– e.g., while out for an evening stroll, he saw a packof identical dogs lined up behind his WestHighland terrier.
7 The Man Who Couldn’t See Motion (cont’d)
• Akinetopsia
– Neurologicical deficit
• caused by disruptions to cortical area of MT